Luminance Dynamic Range is a term used to describe the ratio between the lowest and highest luminance intensities of an image. Digital images that can reproduce a large portion of the luminance dynamic range visible by the human eye are called High Dynamic Range (HDR) images.
In commercial terms, the dynamic range of an electronic imaging device has been known as the “contrast ratio”. It is a measurement of the brightest and darkest aspects of a displayed image. In addition to the ratio between the darkest and lightest outputs that a display can deliver, the level of adjustment between the lowest and highest luminance levels plays a significant part in the quality of a rendered image. There are many ways of calculating the contrast ratio or dynamic range of a display device. As used herein the term contrast ratio (CR) is used to refer to the ratio between the brightest output Imax of a display device and the lowest output Imin of a display device. As such
  CR  =                    I        max                    I        min              .  The term Dynamic Range (DR) is used to refer to the number of transmission levels that a display device has between the lowest luminance level Imin and the brightest luminance level Imax.
Recent developments in display technology have realised High-Dynamic Range displays that can display a range of light that is 30 times brighter and 10 times darker than conventional computer displays. However, the dynamic range of the human eye is far greater than current display technologies can achieve and even the best known display devices achieve a contrast ratio or dynamic range that is considerably below the visual capabilities of the human eye. To improve digital images it is required to achieve display devices with better contrast ratio and/or dynamic range performance.
In US 2008/0174614 to Dolby a display device is disclosed which comprises a backlight source and two spatial light modulators. The two light modulators are disposed in front of the light source. The two light modulators are provided by LCD panels. The luminance at a point on the screen of the display device is determined by the intensity of light incident upon the front-most LCD panel and the degree to which the LCD panel at that point absorbs light being transmitted through it. Typically, LCD panels have a transparency in the range 3-8% even when switched to “white” and so most light energy is actually absorbed.
In WO 2003/077013 to The University of British Columbia a similar display to that described in US 2008/0174614 is shown wherein the backlight source and first modulator are provided by a single matrix of LEDs. Due to the size of the LEDs, their number is much lower than the number of pixels of the front-most LCD panel. This together with the light scattering between the LED panel and the front-most LCD panel can, disadvantageously, generate visible artefacts on the display screen which degrade the rendered images.
Another display device is described in US 2008/0088647 to Apple Inc. The display device disclosed therein has a single bright back-light source and two panels such as LCD light modulating panels. A table is generated of the possible luminance levels. The display is said to feature an extremely high contrast ratio due to the ranges of possible transmission levels at the pixel level of the first and second panels. In US 2008/0088647 a luminance transfer function for the display device is given as:G(i,j)=Y(0)×Ta(i)×Tb(j)×C 
wherein Y(0) is the luminance level of the back-light source; C is a constant and G(i,j) is the luminance level corresponding to transmission levels Ta and Tb of the first and second panels respectively.
The transfer function calculates the maximum number of transmission levels that in combination the two LCD panels can achieve (which includes non-unique transmission levels). As such the above calculation provides an indication of the maximum possible dynamic range of the display device disclosed.
Limitations of the presently known technology include: limitations in the brightness of the display where most of the source-light is attenuated by an LCD modulator, high power consumption of the display devices, screen-size restrictions, contrast ratio and dynamic range performance, fragility of the display and thinness of the display.
High power consumption of the display devices can result in their over-heating. As a consequence, the time that the device can be used is restricted and hence its potential applications. Although coolant can be used to minimise the effects of the high energy-consumption, it does not solve the over-heating problem. Use of coolant is also impractical for many applications and limits the possible applications of such HDR displays.
Yet a further limitation of some known displays is their size. The structure of the current HDR display limits their production on a small scale in all three dimensions, this can limit their application in mobile or hand held devices and in devices where a thinner screen is beneficial. Furthermore, for such and other applications, it is desirable that a display device be flexible.
The present invention seeks to mitigate these and other problems associated with the prior art and seeks to provide an improvement in known display technologies.